1. Field of the Invention
This invention relates to a plasma processing method and a plasma processing apparatus. More particularly, this invention relates to a plasma processing method and apparatus that can form non-single-crystal functional deposited films used as semiconductor layers of semiconductor devices such as, in particular, photoelectric transducers including electrophotographic photosensitive members, image input line sensors or image pick-up devices, optical input devices as typified by photovoltaic devices usable as solar cells, and active devices such as diodes and TFTs (thin film transistors).
2. Related Background Art
Vacuum processing apparatuses as typified by plasma processing apparatuses used in the fabrication of semiconductor devices are used under appropriate conditions adapted to various processing methods carried out according to respective steps and uses. For example, in film formation, apparatuses and methods making the most of their features are used in various manners such that oxide films, nitride films or amorphous silicon semiconductor films are formed by plasma CVD using a plasma CVD apparatus, metal-wiring films are formed by sputtering using a sputtering apparatus, and fine patterns are formed by dry etching using an etching apparatus.
In recent years, there is also an increasing demand for improvements in film quality of films formed and their processing capacity, and various means therefor have been studied.
In particular, plasma processing that utilizes a high-frequency power is preferably employed in many fields because of its various advantages. For example, it ensures a high discharge stability and can be used also in forming insulating materials such as oxide films and nitride films.
Hitherto, the oscillation frequency assigned to high-frequency power sources for discharging commonly used in plasma processing such as plasma CVD is 13.56 MHz. An example of such conventional plasma CVD apparatuses commonly in wide use to form deposited films is shown in FIG. 1. The plasma CVD apparatus diagrammatically illustrated in FIG. 1 is a film forming apparatus suitable when an amorphous silicon film (hereinafter "a-Si film") is formed on a cylindrical substrate for an electrophotographic photosensitive member.
The formation of an a-Si film by the use of this apparatus will be described below.
A reaction chamber (a deposition chamber) 301 that can be evacuated has a cylindrical second electrode 306 and is provided with a cylindrical first electrode constituting a film forming substrate (a substrate for an electrophotographic photosensitive member) 302 as an opposing electrode set opposingly to the second electrode 306. To both ends of the film forming substrate (first electrode) 302, auxiliary substrates 307 and 308 are respectively fitted to constitute part of the first electrode. In order to improve uniformity in film thickness and film properties, the dimension of the second electrode 306 in its cylinder shaft direction is set substantially the same as the total of the lengths of the film forming substrate (first electrode) 302 and auxiliary substrates 307 and 308 in their cylinder shaft direction. The first electrode, film forming substrate 302 is heated from its inside by means of an internal heating element 303. A high-frequency power source 312 is only single-point connected to the second electrode 306 by a cable 313 through a matching circuit 311. The deposition chamber 301 is provided with a vacuum exhaust tube 305 and is connected to an exhaust means (not shown) such as a vacuum pump via a main valve 304. The deposition chamber 301 is also provided with a material gas inlet 309 from which a material gas is fed into the chamber via a material gas feed valve 310.
Films are formed, for example, in the following way.
The film forming substrate 302 serving also as the first electrode is placed inside the deposition chamber 301, and then the main valve 304 is opened to evacuate the inside of the deposition chamber 301 through the evacuation tube 305. Thereafter, the material gas feed valve 310 is opened to feed an inert gas into the chamber through the material gas inlet 309, and its flow rate is adjusted to provide a given pressure. The heating element 303 is electrified to heat the film forming substrate 302 to a desired temperature of from 100.degree. C. to 400.degree. C.
Thereafter, a film forming material gas, e.g., a desired material gas selected from silane gas, disilane gas, methane gas, ethane gas and so forth, and also optionally a doping gas such as diborane gas or phosphine gas, are fed into the deposition chamber 301 through the material gas feed valve 310, and the exhaust velocity is adjusted to maintain the pressure at tens of Torr to hundreds of mTorr.
A high-frequency power of 13.56 MHz is supplied from the high-frequency power source 312 to the second electrode 306 single-point connected to the second electrode 306 by the cable 313 through the matching circuit 311, to generate plasma discharge between the second electrode 306 and the film forming substrate 302, where the material gases fed are decomposed to deposit an a-Si film on the film forming substrate 302 serving also as the first electrode. In the course of this deposition, the first electrode is heated by the heating element 303 to about 100.degree. C. to 400.degree. C.
If necessary, the film forming substrate may be rotated by means of a rotating mechanism (not shown) so that the film thickness distribution in the peripheral direction can be improved.
In this film forming process, the deposition rate for obtaining a-Si films satisfying the performance of electrophotographic photosensitive members is a deposition rate of, e.g., about 0.5 to 6 .mu.m per hour. If the deposition rate is made higher than that, the properties required for photosensitive members may be diminished, or no desired performance can be achieved in some instances. Also, when a-Si films are utilized in electrophotographic photosensitive members, it is commonly considered necessary for the films to be in a thickness of at least 20 .mu.m to 30 .mu.m in order to obtain a chargeability. Hence, it has taken a long time to produce electrophotographic photosensitive members. Thus, a technique that can produce them in a shorter time without lowering the properties required for photosensitive members has been sought.
A report on a plasma CVD method making use of a high-frequency power source of 20 MHz or more, carried out using a parallel-plate plasma CVD apparatus (Plasma Chemistry and Plasma Processing, Vol. 7, No. 3, 1987, pp. 267-273), shows a possibility that the deposition rate can be improved without lowering the performance of deposited film when the discharge frequency is made higher than the conventional frequency 13.56 MHz, and has attracted notice. Making the discharge frequency higher in this way is also reported in sputtering techniques, and in recent years its advantage has been extensively studied.
Now, when films are formed using the above plasma CVD apparatus and in the same manner as the conventional method except that the power is replaced with a high-frequency power of a discharge frequency higher than the conventional frequency 13.56 MHz, it is certainly possible to ascertain that films can be formed in a higher deposition rate than conventional cases. In such a case, however, it has been found that the following problems may occur which have not been questioned in the case of the discharge frequency of 13.56 MHz.
That is, when films are formed while rotating the film forming substrate 302, it is true that films having substantially the same properties as those of conventional films can be deposited and also as a matter of course can be deposited in a uniform film thickness distribution in the peripheral direction. However, it has been found that the film thickness becomes uneven in the peripheral direction and great uneven film thickness distribution occurs when films are formed when the film forming substrate is set stationary, as when using a deposited film forming apparatus from which the mechanism for rotating the film forming substrate is omitted or without operating the rotating mechanism so that the apparatus can enjoy a cost reduction and its maintenance can be made less troublesome. More specifically, it has become clear that, in practice, the state of plasma generated in the apparatus is fairly localized in the peripheral direction and also the deposition rate greatly differs at some portions. Such an uneven film thickness distribution is considered not to occur on the surface when films are formed while rotating the film forming substrate.
It has been also found that what differs in the peripheral direction is not only the deposition rate, but also the electrophotographic performance of deposited films, the latter being fairly different at some portions. This uneven performance cannot be explained from only the difference in thickness of deposited films, and hence the film quality itself of the deposited film is presumed to be different in its peripheral direction. It has been also found that the photosensitive member has a better performance at portions with a good film quality in the peripheral direction than a photosensitive member on which the film has been formed while rotating the film forming substrate, and on the other hand has a poorer performance at portions with a poor film quality in the peripheral direction than the photosensitive member on which the film has been formed while rotating the film forming substrate. Namely, in the photosensitive member on which the film has been formed while rotating the film forming substrate, it is presumed that a film with poor properties and a film with good properties are formed in layers and hence an averaged performance is exhibited.
As discussed above, in the film formation using a high-frequency power of a frequency higher than the conventional 13.56 MHz, the film thickness distribution and film properties become uneven in the peripheral direction when films are formed when the film forming substrate is stationary. As a result, in workpieces with a relatively large area, such as electrophotographic photosensitive members, uneven images, problematic in practical use, have often occurred.
In the case when films are formed while rotating the substrate to be processed, the film with good properties and the film with poor properties are inevitably formed in layers because of the localization of plasma, so that film properties totally deteriorate so that good film properties cannot be obtained which should have been.
Such uneven deposition rate and film properties may cause a great problem when for functional deposited films formed of non-single-crystal materials, including amorphous or microcrystalline materials, which are useful not only for electrophotographic photosensitive members but also image input line sensors, image pick-up devices, photovoltaic devices and so forth. Also, in the case of other plasma processing such as dry etching and sputtering, similar uneven processing may result when the discharge frequency is made higher, and may cause a great problem in practical use.